The Arp2/3 complex nucleates new filaments in site-directed, signal-controlled fashion at the leading edge of motile cells. This gives rise to the formation of dense, branched actin networks that are thought to provide the force that drives lamellipodia protrusion. The studies proposed here aim at determining the three-dimensional structure and structural changes of the Arp2/3 complex occurring during activation and branch junction formation. A combination of electron cryomicroscopy and various image analysis approaches will be used to provide three-dimensional reconstructions of theArp2/3 complex in its free, activated and actin-bound forms. This will be complemented by subunit localization using gold labeling. As the filamentous nature of F-actin precludes conventional crystallographic methods, the above approaches provide the only link between structural information of the individual proteins and their structure in the actin-bound forms at various levels of structural complexity. Docking of the crystal structures into the electron microscopic reconstructions will allow to correlate atomic resolution information of the individual components to their multi-complex form.